Start of by making your colourful objects. I used jewels for mine. You can zoom in to do it, but make sure you have most the page filled up with your objects.

Step 2

Mine ended up like this.
-WARNING- this can take a long time if you actually make each object. The best thing to do is make a row, then copy and paste it, and move it into postition.

Step 3

Next, us the smudge tool to get the nice effect we are after. Start by going straight down from the top of the page, then for the second line, go straight up from the bottom, and so on. I used:
Size: 32
Opacity: 128
Colour: 0

Step 4

It should end up like this. Then you're done! it's a perfect, colourful background.